Taw Richardson, chief executive of the drug’s manufacturer, AgroSource, said his company’s research found the drug to be effective. He questioned the methodology of the new study, saying that researchers used only one delivery method for the drug and not the range of products that the company recommends for ensuring it penetrates the leaves.

But environmentalists and public health advocates said the results suggested that the E.P.A.’s approval of oxytetracycline was based on flawed data, which was provided by Agrosource.

“It’s pretty frustrating because they are creating a huge environmental risk and risk for people applying the drug,” said Steven Roach, the director of food safety programs at Food Animal Concerns Trust, an advocacy group. “This is what happens when you have an agency that responds in a panic mode. It makes me worry about how the E.P.A. is making decisions on antibiotics.”

Asked to comment on the study, the E.P.A. did not provide a response.

Christopher Vincent, a physiologist at the University of Florida who studies citrus greening, said the waxy coating on the leaves of orange trees has been an impediment to delivering oxytetracycline into the plant’s phloem, or vascular system. A study he published earlier this year with a group of scientists that included Professor Wang, found the drug does penetrate the leaves, though at relatively low levels.

“It’s been difficult to know how much of your application is getting into the phloem, but this new paper gets closer to answering that question than anyone has gotten before,” he said.

The drug is one of two human-grade antibiotics that the agency has approved to treat citrus greening, which now threatens commercial groves in California and other citrus-growing states. In humans, the drugs, oxytetracycline and streptomycin, are used to treat pneumonia, syphilis and a broad array of infections. In 2016, the compounds were approved for emergency use on citrus trees in Florida, and in December, the agency expanded the use of oxytetracycline for orange groves across the country. A wider rollout for streptomycin is still pending.

The approvals were made over objections from the Food and Drug Administration and the Centers for Disease Control and Prevention, which said expanded use of the drugs could encourage dangerous bacteria to mutate to survive the drugs and infect humans with pathogens that are impervious to existing antibiotics.